ABSTRACT: Clay Paragenesis and Petrology of Sandstone Reservoirs, Fuller Reservoir Field, Wind River Basin, Wyoming

THOREZ, J., Liege State University, Liege, Belgium, D. BOSSIROY, INIEX, Liege, Belgium, and R. M. FLORES and C. W. KEIGHIN, U.S. Geological Survey, Denver, CO

Facies analyses of cores from the Paleocene Fort Union Formation in Fuller Reservoir field, Wind River basin, Wyoming, recognized fluvial-channel, crevasse-splay, delta-front, and offshore-lacustrine sandstone reservoirs. Clay minerals of these facies reservoir types were analyzed to determine their paragenesis. Petrographic examination of reservoir, and adjacent sandstones shows that the sandstones were diagenetically modified by moderate to extensive dissolution of plagioclase and other framework grains, and by formation of clay minerals, especially kaolinite and illite.

The <2-m clay fraction was extracted from these sandstone reservoirs and prepared as oriented aggregates. The samples were X-rayed in air dried, glycolated, and heated (325 degrees -500 degrees C) states. Complementary tests, including attack with boiling 4N HCI followed by hydrazine saturation (kaolinite "crystallinity") and Li-saturation for identification of swelling components, were performed to yield more accurate qualitative analyses. X-ray diffraction analyses indicated a complex clay composition with a predominant kaolinite component that comprised well-crystallized kaolinite (Kt), disordered kaolinite (Kd), and dickite (D). Minor components were chlorite (C) and illite (I). Chlorite consisted of degraded Fe-chlorite or neoformed Mg-chlorite. Illite consisted of mixed layer illite-smectite (I/S) containing less than 50 % swelling layers. These clays are a combination of illite with montmorillonite (cf. I/M) and illite with beidellite interlayers (cf. I/Bei); the former represents neoformed "hairy" illite in sandstone pores, whereas the latter reflects a transitional degradational stage in the weathering of parent (feldspar) silicate structure. Also present were traces of mixed-layer (10-14C) material that was made up of illite layers and distended interlayers behaving like chlorite.

Although porosity in the reservoir sandstones is often good, it is quite variable, ranging from 0.3 to 27 modal percent. Many pores are partially to extensively occluded by pore-filling clays, especially kaolinite. Locally, porosity has been virtually eliminated through precipitation of iron-bearing carbonate cement.

Quantitative analyses of clay components do not indicate a relationship between clay paragenesis and reservoir types. This may be due to complex diagenetic history, which includes provenance-inherited clays (I, Kd, C, and 10-14C), in situ weathering (I-Bei), and in situ neoformation (I/M with "hairy" illite, Kt, D, and Mg-chlorite). Modal analyses reveal moderate scatter in distribution of quartz, feldspar, and rock fragment framework grains in the sandstone. This uneven distribution of framework grains probably is a function of the variability of depositional environments.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)